1. Field of the Invention
This invention relates to a Fabry-Perot type optical fiber interferosensor in which light of a low coherence light source is guided to one optical fiber of a sensor having a certain measured clearance, parallel planes are formed, end surfaces of the optical fibers are faced to each other to form partial reflection mirrors at the opposing end surfaces of the optical fibers, either one of a reflection light or through-pass light has a modulated in wavelength in correspondence with a clearance size of the measured clearance and is guided with the optical fibers through multi-reflection at the measured clearance, an optical intensity in time-series corresponding to the variation in clearance is attained with the optical intensity distribution sensor, and either a minimum optical intensity position or a maximum optical intensity position at the optical intensity signal is calculated to measure the measured clearance; and more particularly, an optical fiber interferosensor to perform a signal processing preferable in an optical fiber interfere strain sensor where the measured clearance is varied in response to a displacement of the strain generated segment; a signal processing system for the optical fiber interferosensor; and a recording medium.
2. Description of the Related Art
The optical fiber interferosensor applied with a low coherence light source having a wide range spectrum, i.e. a white light source of multi-wavelengths has a feature not found in the optical fiber interferosensor using a high coherence light source such as a laser light source.
The low coherence interference process is a well-known technology in a classical optics as already described in M. Born and E. Wolf's “Principles of Optics, 6th edition” (Pergamon Press) (Oxford, London, New York, 1980). However, a true value of this process is realized under application to the optical fiber sensor.
In addition, a system in which the low coherence interference process is applied to the optical fiber sensor is already described in a total report such as D. A. Jackson's “Monomode optical fiber interferometers for precision measurement”, J.Phys.E: Sci. Instrum., Vol. 18 981-1001 (1985).
In contrast to the low coherence interference process, a high coherence interference process advances as a laser is developed and then a high precision measurement is realized. In this high coherence interference process, a phase difference, i.e. a difference in optical path is measured basically by counting the number of fringes. Due to this fact, if the measuring system is turned off, information stored in it up to now is lost and an absolute optical path difference may not be obtained.
In addition, the high coherence interference process has as its reference value a wavelength of laser acting as a light source. Accordingly, in the case of high coherence inteference process, the high precision measurement cannot be realized unless the laser wavelength is highly stabilized.
To the contrary, the low coherence inteference process has some features not found in the high coherence interference process that the absolute optical path difference of the interferometer can be measured and it is hardly influenced by variation in the wavelength spectrum in the light source. This process can be applied to many kinds of physical quantities if the measured parameters can be converted into displacements and further this process enables their absolute measurements to be carried out.
That is, the low coherence interference process is a high precision measuring process overcoming a difficulty in identification of the fringe degree in the high coherence interference process and a problem related to stability in operation of a light source while some features as found in the interference process are being utilized.
A Fabry-Perot type optical fiber interferosensor using the low coherence light source selected from the prior art optical fiber interferosensor has a simple structure, so that this is suitable for a high precision sensor.
As the prior art Fabry-Perot type optical fiber interferosensor applied with a low coherence light source, an optical fiber Fabry-Perot type interference strain sensor system is already indicated in the gazette of U.S. Pat. No. 5,392,117 (issued on Feb. 21, 1995; Belleville et al. “Fabry-Perot optical sensing device for measuring a physical parameter” and the gazette of U.S. Pat. No. 5,202,939 in which a signal light from a sensor having modulated in wavelength is collected in a spatial linear manner at the sensor, the light signal is guided to a Fizeau interferometer, an optical correlation is generated at the Fizeau interferometer to demodulate the light signal and this is detected by a linear image sensor.
One example of the Fabry-Perot type optical fiber interference strain sensor system disclosed in the aforesaid gazette of U.S. Pat. No. 5,392,117 will be described.
In FIGS. 27A and 27B are indicated schematic configurations of a Fabry-Perot type optical interference strain sensor. FIG. 27A shows an entire configuration of the Fabry-Perot type optical interference strain sensor and FIG. 27B schematically shows the measuring system.
As shown in FIG. 28, the Fabry-Perot type interferometer forms optical partial reflection films F1, F2 in a pair of parallel flat planes opposing to each other while a predetermined gap clearance size d (=G) being left, wherein a multi-reflection is carried out by performing a repetitive reflection within the gap and a large number of optical fluxes concern the interference (multi-optical flux interference), resulting in that a wavelength selective characteristic of the optical wave is increased. The present invention to be described later also positively utilizes this feature. In this case, a gap clearance (d) is filled with either vacuum or medium such as air, for example, which is transparent against the light source to be used.
A structure of the Fabry-Perot type optical fiber interference strain sensor will be described as follows.
As shown in FIG. 27(a), the strain sensor has a basic structure in which partial reflection films F1, F2 composed of a thin film forming a partial reflection mirror are formed at each of the end surfaces of the optical fibers 101, 102 oppositely faced to each other while a gap clearance size G (=d) is being left. In addition, the optical fibers 101, 102 oppositely faced to each other at the sensor part 100 are inserted into a micro-capillary tube 103, arranged with the predetermined clearance size G being present, and further the fibers are melted and fixed to the micro-capillary tube 103 at the part corresponding to a gauge length LG.
When the micro-capillary tube 103 constituting the Fabry-Perot type optical fiber interference strain sensor is fixed to the measured item with adhesive agent or the like, the gauge length LG is changed in response to deformation of the measured item and correspondingly a
gap clearance size G is changed. A strain of the measured item can be measured in reference to a variation of the clearance size G (FIG. 29). Further, in order to use a low coherence light source having a wide wavelength region, as the optical fibers 101, 102, the multi-mode optical fiber is used.
Although both the reflected optical wave and the transmitted optical wave of which wavelengths are modulated at the sensor part 100 can also be used as a signal light, the reflected light will be described as an example.
The modulated in wavelength optical wave reflected at the sensor part 100 and returned back is divided by a [2×2] coupler 107 and is transmitted to an optical signal demodulation processing part 108.
Then, referring now to FIG. 29, a sensing principle of this type of the Fabry-Perot type optical fiber interference strain sensor will be described.
The signal light of which wavelength is modulated and transmitted from the sensor 100 to the optical signal demodulator 108 is radiated into the spatial space, collimated by an optical system 108a composed of a collimate lens system and a light collecting lens system, for example, thereafter the light is collected in a linear manner (a parallel light beam form).
The light signal collected in a linear manner is multi-reflected at each of the positions of a tapered gap of a wedge type Fabry-Perot interferometer, i.e. a Fizeau interferometer 108b where opposing surfaces are inclined by only a minimum angle γ in a substantial similar manner as that of the aforesaid sensor part 100, an optical correlation with the wavelength spectrum characteristic is generated and the optical intensity becomes minimum at the position of the gap size of the Fizeau interferometer 108b coinciding with the gap clearance size G of the sensor part 100 (in the case of transmittance type, it becomes maximum and in the case of reflection type, it becomes minimum).
This optical intensity signal is detected by a linear image sensor 108c comprised of CCD (Capacitance-Coupled Device) or the like, for example, and a distance from a wedge end of a location where its optical intensity becomes minimum [Lmin(d)|d=G] is calculated, as shown in FIG. 29, thereby an absolute measurement of the clearance size G at the gap of the sensor part 100 can be carried out with [G=Lmin(G)·tan(g)] being applied. A strain ε can be expressed as                     ɛ        =                                                            {                                                                            L                      min                                        ⁡                                          [                                              G                        ⁢                                                                                                   ⁢                                                  (                          ɛ                          )                                                                    ]                                                        -                                                            L                      min                                        [                                          G                      ⁡                                              (                        0                        )                                                                                            }                            ·              tan                        ⁢                                                   ⁢                          (              γ              )                                            L            G                                              (        1        )            
In this system, when a high coherent light source is used as a light source, the optical correlation signal is dispersed to cause either a minimum or maximum position sensing to become difficult.
In FIG. 30 is illustrated an example of simulation of a sensor system output when the central wavelength λ0=850 nm, its full width at half-maximum Δλ is set to 5 nm, 25 nm and 60 nm, respectively. It becomes apparent that as the spectrum full width at half-maximum becomes narrow and the light source becomes a high coherence, the optical correlation signal is expanded, a zero-path length interference region of the Fizeau interferometer 108b is expanded and a minimum position sensing of the optical correlation signal becomes difficult. In FIG. 30, a position of 1.0 at an abscissa corresponds to the gap clearance.
A schematic block diagram of a transmission type system of the aforesaid measuring system is illustrated in FIG. 31 and a schematic block diagram of a reflection type system is shown in FIG. 31, respectively.
In FIGS. 33A and 33B are illustrated an example of output where a signal output in the aforesaid system is simulated. A part enclosed by a dashed line in FIG. 33A corresponds to an optical correlation signal containing a gap clearance information of the sensor part 100. FIG. 33B shows an enlarged optical correlation signal part. A position at 1.0 in the abscissa corresponds to the gap clearance position. In addition, a signal near the zero position adjacent to the wedge end of the Fizeau interferometer 108b enclosed by a one-dotted chain line in FIG. 33A is a signal generated only by the Fizeau interferometer 108b and since it uses a low coherence light source, it is locally present near the position where the gap clearance of the Fizeau interferometer 108b is zero. This is not a signal generated by an optical correlation between the sensor part 100 and the Fizeau interferometer 108b. This signal portion is defined as a zero-pass length interference region.
In FIGS. 34A and 34B are shown measurement block diagrams of a comparison confirmation test about a difference between the optical correlation signal and the zero-pass length interference. In FIG. 34, the gap fixed Fabry-Perot interferometer shown in FIG. 34B is a Fabry-Perot interferometer in which after partial reflection films are formed at the end surfaces of the two optical fibers, they are inserted into micro-capillary tubes, their end surfaces are faced and fixed to each other with a gap being present between them, wherein it has a structure similar to that of the aforesaid Fabry-Perot type optical fiber interference strain sensor shown in FIG. 27 and this may perform to act as a sensor. The gap-variable type Fabry-Perot interferometer shown in FIGS. 34A and 34B are made such that after the partial reflection films (TiO2 thin film, for example) are formed at the end surfaces of the two optical fibers, each of them is faced against a piezo-stage, connected and fixed to each other, its gap clearance is made variable so as to act as a Fizeau interferometer 108b of the optical signal demodulation part 108 shown in FIG. 27A. That is, the Fizeau interferometer 108b operates to measure signal light from the sensor 100 once with an interferometer having a wedge while being spatially expanded and in turn the gap variable type Fabry-Perot interferometer is operated to measure for every gap clearance size while the gap clearance size is being changed in sequence. In this case, as the light source, LED (light emitting diode)(L7560[HAMAMATSU]) was used, as the optical fiber, a multi-mode fiber (GI50/125, NA=0.12) was used and as a piezo-stage, a three-axis nano positioning stage (17ANC001/MD[MELLESGRIOT]) was used.
In FIGS. 35A and 35B are illustrated a variation in output of the optical power meter in the two measuring systems in FIGS. 34A and 34B in the case that the measurement was performed while the gap clearance size of the gap variable Fabry-Perot interferometer being minutely changed at the piezo-stage. As described above, the zero-pass length interference region where the gap clearance size of the gap variable Fabry-Perot interferometer is near zero is present at any type of measuring system, although the optical correlation signal is present only when the gap fixed Fabry-Perot interferometer acting as a sensor is present, and the minimum position of the signal corresponds to the gap clearance size of the gap fixed type Fabry-Perot interferometer. A result similar to these systems can be indicated also by simulation.
In order to attain the output signals indicated in FIGS. 40A, 40B in the practical sensor measuring system and realize a high precision measuring system, it is necessary to attain, as signal output waveforms of time-series in the linear image sensor, a clear waveform having a less amount of fluctuation and variation of a low frequency as well as a high S/N ratio (a signal to noise ratio).
In order to realize the foregoing, it becomes necessary to perform an adjustment (a fine adjustment) of high precision alignment in the optical system including a collimate lens, a focusing lens, a Fizeau interferometer and a linear image sensor and the like. Further, the linear image sensor shows a tendency that a high sensing noise is generated and the sensing system may become complicated.
In view of the foregoing, it may be considered to realize the high-precision sensor measuring system by attaining a signal acting as a background having the optical correlation signal separated from the measuring range (a rated range of the sensor), performing a series of signal processing on the basis of that data and removing non-required signals such as fluctuation and noise of low frequency or the like. In this optical fiber interference sensor, a load characteristic data of wider range than the measuring range at the time of calibration is attained and as data, the data of measurement range is utilized as background data. This method has some effects that a high precision measuring system can be realized by an adjustment of a convenient optical system and the cost can be reduced.
However, in this system, if the wavelength spectrum regions in the light sources are the same to each other, the output signal waveforms of the sensor system are scarcely changed. That is, in FIGS. 36A to 38 are indicated examples of simulation of the sensor system output signal waveforms (FIGS. 37 and 38) from the light source having four different wavelength spectra distribution (FIG. 36) where the light source wavelength spectrum regions are approximately the same to each other. In addition, this fact is also confirmed in an actual experiment as shown in FIGS. 39A, 39B and 40.
That is, FIGS. 36A, 36B, 36C and 36D show a result of simulation of a wavelength spectrum distribution, a reflection signal spectrum from a sensor and a wavelength spectrum after passing through the variable gap Fabry-Perot interferometer for the light source 1, the light source 2, the light source 3 and the light source 4 having four different wavelength spectrum distributions with substantial same light source wavelength spectrum regions. In view of the wavelength spectrum, the waveforms in regard to these four light sources (the light source 1 to the light source 4) are made substantially different from each other. Then, (a), (b), (c) and (d) in FIG. 37 and (a), (b), (c) and (d) in FIG. 38 illustrate results of simulation of light intensity outputs against the gap clearances of the signal lights from the light source 1 to the light source 4 passed through the variable gap Fabry-Perot interferometer. In this case, the sensor gap clearance was 20 μm. As apparent from FIG. 38, irrespective of the fact that the wavelength spectrum shapes of the light source 1 to the light source 4 are substantially made different, the optical correlation signal waveforms in regard to the light source 1 to the light source 4 are remarkably similar to each other.
FIGS. 39A and 39B illustrate a wavelength spectrum distribution of the two LED light sources (L7560[HAMAMATSU] and RLE8P4-002[DAIDO STEEL CO.,LTD.], and FIGS. 40A and 40B illustrate a result of measurement performed by experiment of these optical correlation signals. The wavelength spectrum of RLE8P4-002 is shifted to a long wavelength side as compared with that of the other L7560, a spread of the spectrum is slightly narrow, so that the corresponding optical correlation signal waveforms are spread, although they are quite similar optical correlation output waveforms irrespective of the fact that the spectrum waveforms are different from each other.
As described above, the fact that the quite similar optical correlation output waveforms can be attained irrespective of different spectrum waveform becomes a quite important factor in view of realizing a high precision sensor. That is, even if the light source wavelength spectrum is varied by a certain degree due to an external disturbance such as a temperature or the like, the optical correlation signal waveform is not changed and a gap clearance position of the sensors can be measured in a stable manner. Further, a wide setting of the wavelength spectrum distribution of the light source causes the optical correlation signal not to be expanded and influence of non-required noise component is reduced. Accordingly, it is not necessary to provide a high stable state of the light source and it becomes possible to realize the high precision sensor system in less-expensive manner.